Generally a catalyst is evaluated not only in terms of its activity but also in terms of its useful life. A catalyst, even though excellent in activity, is practically useless if it has a short life. It is also useless if it is easily breakable and therefore short in mechanical life.
Usually for the manufacture of a high-activity, long-life catalyst, a catalyst carrier is required to meet many conditions. For instance, to be usable in an exhaust gas purifying catalyst device for rendering harmful elements contained in auto engine emission gases such as carbon monoxide, unburnt hydrocarbons and nitrogen oxides harmless, the catalyst is required to meet the following conditions.
First, since the temperature in the engine during combustion becomes higher than 800.degree. C., the catalyst used must be one that exhibits only a small decrease in activity even at high temperatures. Besides, it must be strong enough to withstand the vibrational forces generated in auto operation.
Automotive fuel containing chemical compounds such as lead, sulphur and phosphorus naturally produces exhaust gases containing lead compounds, sulphur compounds and phosphorus compounds. It is equally natural that since the lubricating oil for the internal combustion engine containing phosphorus compounds or the like which is, in most cases, burned in the internal combustion engine, the exhaust gas also contains elements attributable to the lubricating oil. Among others, the lead compounds, the sulphur compounds and the phosphorus compounds are commonly found harmful to the catalyst. Therefore the catalyst is required to have an adequate resistance to these harmful substances.
Generally speaking, the mechanical strength and the above-mentioned resistance to the harmful substances contained in the exhaust gas depend largely on the carrier for the catalyst. For this reason the carrier for the exhaust gas purifying catalyst has to be strong enough to resist both thermal shock and mechanical vibration and also sufficiently resistant to the harmful contents of the exhaust gas.